Matrix printing systems are systems in which all the necessary characters or images are printed as groupings of small, closely spaced dots or line segments which, when seen from a normal viewing distance, blend to form the desired visual effect. Considerable research effort has been directed to the development of a reliable, relatively inexpensive, high speed non-impact electrostatic matrix printing system for converting data or imagery in the form of electrical signals into permanent form with high accuracy and resolution, and without the need for special paper or other substrate material, or exotic toners and image developers. The present invention is a step forward in this development effort, and is an advance over the invention disclosed in commonly assigned U.S. Pat. Application Ser. No. 053,853, filed July 2, 1979 now U.S. Pat. No. 4,246,839.
In the past, non-impact printing of data has been accomplished in several ways. For example, in one matrix system, an electrostatic image is imprinted on the surface of a highly insulating substrate through the use of an array of high voltage electrodes in close proximity to the substrate surface. The image is then developed by allowing toner particles charged in opposite polarity to coat the electrostatically imaged surface, and fixing those particles attracted to the image through thermal, chemical or other means. In a variation of this system, an array of electrodes is placed opposite an ink or particle source, with a substrate lying therebetween. An electrical potential selectively placed on electrodes in the array produces a field which penetrates the substrate and causes non-aqueous ink or toner particles to be attracted from the source to the surface of the substrate opposite the electrode. Variations of these systems include using corona discharges to produce ink-attracting images on paper or other substrates, using highly insulating or photoconductive or other specially constructed belts to act as image transfer means, and varying the positions of imaging means and ink or particle sources relative to the substrate. It is also known in the art to use a barrier coating or film on the substrate or paper as a means of either blocking developer from access to areas on the substrate which have already been charged or as a means of preserving the existing charge in an imaged area and thereby attracting the ink or developing material.
In spite of intensive efforts in this art, major shortcomings in known non-impact printing processes have remained, especially in connection with electrographic processes, i.e., processes which must transform real time electrical impulses into imagery, as opposed to photographic or facsimile processes which merely duplicate text or images from a copy master, such as are used in office photocopying machines. One such major shortcoming involves the phenomenon known as "background", i.e., the presence of undesired dots, lines, or shaded background areas in printed copies. Attempts to overcome this problem have often involved the use of relatively expensive pre-treatment of the paper or other substrate used as the print medium. Another common problem has been maintaining uniformly satisfactory print quality. Specialized, expensive inks or dyes have been found necessary in some systems to produce a satisfactory image or to maintain proper print quality, particularly where the printed image has required the even application of ink over a relatively large area. Yet another problem has been the relative lack of speed commonly available in present plain paper electrographic systems.
The printing system described in commonly assigned U.S. Pat. No. 4,246,839 made a significant contribution towards solving these problems in an effective and economical way. It was found, however, that under certain conditions, some difficulty was experienced with that system in connection with maintaining overall print quality and completely eliminating unwanted background. It has been discovered that, when using plain non-dielectric paper as a substrate, print quality is affected by the amount of atmospheric moisture which has been absorbed by the paper. This finding is contrary to the teachings in the available literature, wherein substrate moisture is regarded as either unimportant or as a positive benefit. For example, in many cases where electrographic paper is used, the moisture serves to enhance the coupling of the electrostatic field from the electrode through the paper.
Applicant discovered that the rapid passage of paper over the insulating matrix used to support the individual styli in the stylus bar tends to induce the generation and accumulation of a triboelectric charge on the stylus bar and paper, particularly when the stylus tips are not raised above the surface of the matrix. This triboelectric charge build-up in the immediate vicinity of the styli causes severe background and degradation in print quality. In addition, flush or recessed mounting of the stylus tips does not allow the paper to achieve contact with the stylus tips, often resulting in sporadic print quality. It has been found that when the tips of the styli are raised slightly above the insulating matrix to reduce the aforementioned effects, the paper tends to wrinkle as it is drawn over the stylus tips and thereby loses the uniformly close contact with the stylus tips which is associated with consistently high print quality. The apparatus and method of the invention described herein represents a substantial advancement in the art of non-impact electrographic printing, in that it effectively overcomes the problems discussed above.